Disclosed here is an IGFET formed on the single crystal silicon substrate where the major plane surface is deviated within the range from 22 degree to 34 degree toward the crystallographic surface {1,1,1} from {1,0,0} or on the silicon epitaxial layer formed on said substrate. Here, generation of silicon nitride is suppressed, which is newly formed under the mask in the selective oxidation process using the silicon nitride as the mask and also is the main cause of lowering the breakdown voltage of the gate insulating film. In addition, various kinds of functional characteristics depending on the crystallographic surface orientation are not interfered at all. Thereby, the present invention can offer an IGFET which drastically improved the breakdown voltage failure rate of the gate insulating film while keeping the functional characteristics at the best condition.
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1. A method of manufacturing an insulated gate field effect transistor comprising the steps of
forming a silicon dioxide film on a single crystal silicon substrate having a surface deviated by an angle within a range of from 30.3° to 34° from the {100} crystallographic surface toward the {111} crystallographic surface, forming a silicon nitride film on said silicon dioxide film, patterning said silicon nitride film to include a portion remaining on a forming area for said insulated gate field effect transistor on said substrate wherein said forming area includes areas corresponding to the source and drain regions of said insulated gate field effect transistor, forming a thermally oxidized film selectively on and embedded in said substrate using the remaining silicon nitride film as a mask, removing said remaining silicon nitride film from said silicon dioxide film, removing said silicon dioxide film on said forming area of said substrate, forming a gate insulating film on said surface of said substrate in said forming area, said thermally oxidized film having a greater thickness than said gate insulating film, forming a gate electrode on said gate insulating film for providing an electric field to said substrate across said gate insulating film; and forming said source and drain regions using said gate electrode as a mask.
10. A method of manufacturing an insulated gate field effect transistor comprising the steps of
forming a silicon dioxide film on a single crystal silicon substrate of a first conductivity type and having a surface deviated by an angle within the range from above 30.3° to substantially 34° with respect to the {100} crystallographic surface towards the {111} crystallographic surface, forming a silicon nitride film on said silicon dioxide film, patterning said silicon nitride film to remain on a forming area for said insulated gate field effect transitor on said substrate by photoetching, injecting ions of said first conductivity type into said surface of said substrate using the remaining silicon nitride film as a mask to form a channel stop region, forming a thick silicon dioxide film by thermal oxidation using said remaining silicon nitride film as a mask, removing said remaining silicon nitride film on said silicon dioxide film, removing said silicon dioxide film on said forming region of said substrate, forming a gate insulating film on said single crystal silicon substrate in said forming region, forming a polycrystalline silicon film on said gate insulating film, patterning said polycrystalline silicon film to remain as a gate electrode on said gate insulating film by photoetching, selectively implanting ions of the opposite conductivity type into said single crystal silicon substrate using said gate electrode as a mask, forming source and drain regions having said opposite conductivity type by activating the implanted ions, forming a phosphorus silicate glass film on the upper surface of said substrate, providing windows for contacting said source and drain regions by photoetching said phosphorus silicate glass film and said gate insulating film, and providing electrodes on said phosphorus silicate glass film for contacting said source and drain regions.
11. A method of manufacturing an insulated gate field effect transistor comprising the steps of
forming a silicon epitaxial layer with a first conductivity type on a single crystal silicon substrate having a surface deviated by an angle within the range from above 30.3° to substantially 34°, with respect to the {100} crystallographic surface towards the {111} crystallographic surface, forming a silicon dioxide film on said silicon epitaxial layer, forming a silicon nitride film on said silicon dioxide film, patterning said silicon nitride film to remain on a forming area of said epitaxial layer for said insulated gate field effect transistor by photoetching, injecting ions of said first conductivity type into the surface of said silicon epitaxial layer using the remaining silicon nitride film as a mask to form a channel stop region, forming a thick silicon dioxide film by thermal oxidation on and embedded in said epitaxial layer using the remaining silicon nitride film as a mask for thermal oxidation, removing the remaining silicon nitride film from said silicon dioxide film, removing said silicon dioxide film on said forming area, forming a gate insulating film on said silicon epitaxial layer in said forming area, forming a polycrystalline silicon film on said gate insulating film, patterning said polycrystalline silicon film to provide a gate electrode on said gate insulating film by photoetching, implanting ions of the opposite conductivity type into an area of said silicon epitaxial layer through said gate insulating film using said gate electrode as a mask, forming source and drain regions having said opposite conductivity type in said silicon epitaxial layer by activating the implanted ions, forming a phosphorus silicate glass film on the upper surface of said substrate, providing windows for contacting said source and drain regions by photoetching said gate insulating film, and providing electrodes on said phosphorus silicate glass film for contacting said source and drain regions.
2. The method of
4. The method of
selecting said single crystal silicon substrate have a first conductivity type, and forming source and drain regions in said substrate on each side of said gate insulating film with the opposite conductivity type.
6. The method of
selecting said single crystal silicon substrate to have a first conductivity type, implanting ions of the opposite conductivity type into said single-crystal substrate adjacent said gate insulating film, and forming source and drain regions having said opposite conductivity type by activating the implanted ions.
7. The method of
8. The method of
selecting said single crystal silicon substrate to have a first conductivity type and, injecting impurities of said first conductivity type into said surface of said substrate using said remaining silicon nitride film as a mask to form a channel stop region.
9. The method of
selecting said single crystal silicon substrate to have a first conductivity type, implanting ions of the opposite conductivity type into said single-crystal substrate adjacent said gate insulating film, and forming said source and drain regions having said opposite conductivity type by activating the implanted ions.
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This is a continuation of application Ser. No. 215,822 filed Dec. 12, 1980.
The present invention relates to an insulated gate field effect transistor (IGFET), particularly to an IGFET which has been subjected to the selective oxidation process with the silicon nitride film (Si3 N4 film) used as the maks.
The IGFET which has been subjected to the selective oxidation process using the Si3 N4 film as the mask shows less parastic channel effect since the silicon dioxide film (SiO2 film) on the area other than the gate is sufficiently thick. On the other hand, when it is adopted especially to an integrated circuit (IC), it provides excellent effects such as the integration density can be much improved and occurrence of disconnection in wiring is reduced. However, simultaneously, the selective oxidation process brings about possibility of dielectric break down of the insulated gate film at a low voltage when the electric field is applied to the substrate from the gate electrode via the insulated gate film. Namely, such IGFET has a problem that the break down voltage of the insulated gate film is lowered by the abovementioned selective oxidation process.
FIG. 1 to FIG. 4 show formation of field oxidized film by the selective oxidation in the ordinary IGFET and the causes of lowered break down voltage of the insulated gate film which are currently considered.
At first, as indicated in FIG. 1, the silicon dioxide film 2 (SiO2 film) is formed in the thickness of about 500 Å on the entire surface of single crystal silicon substrate 1, and then the silicon nitride film (Si3 N4 film) 3 is selectively formed in the similar thickness on said SiO2 film 2. Next, as indicated in FIG. 2a, the field oxide film 22 is caused to grow up to the thickness of about 8000 Å on the single crystal silicon substrate 1 except for the gate under a high temperature oxidizing atomosphere containing water vapor. However, in this selective oxidation proces, H2 O in the water vapor and the Si3 N4 film 3 react chemically as expressed by the reaction (1), producing NH3.
Si3 N4 +6H2 O=4NH3 +3SiO2 . . . (1)
The ammonia (NH3) easily passes through the SiO2 film and therefore the NH3 generated by the reaction (1) reaches the area under the SiO2 film 2 of the gate portion and then reacts with the single crystal silicon substrate 1 in accordance with the reaction (2), producing Si3 N4 21, 23.
3Si+4NH3 =Si3 N4 +6H2 . . . (2)
Among the Si3 N4 21, 23 newly produced by the reaction (2), the Si3 N4 21 at the boundary of the single crystal silicon substrate 1 under the end of the oxidation resistant mask Si3 N4 3 is called the "White Ribbon". As indicated in FIG. 2b which shows an enlarged view of a part of FIG. 2a, this Si3 N4 21 is generated by the seepage of H2 O from the end of thick SiO2 film. Detail explanation is omitted here since it is explained by E. Kooi et al. in the Journal of Electro-Chemical Society Vol. 123, P117 (1976). In addition it is also explained by Kowada et al. in the Journal of Japanese Applied Physics Vol. 17, No. 4, P737 (1978) that the break down voltage of the insulated gate film is not lowered only by the Si3 N4 21 at the gate end region, the Si3 N4 has a possibility of existing in the gate center region. The reason is considered as follow, namely the NH3 generated by the reaction between H2 O and Si3 N4 because of the crystal defect such as a pin-hole in the Si3 N4 film 3 further reaches the Si substrate passing through the lower SiO2 film 2, producing Si3 N4 23. In addition, as indicated in FIG. 3, in the ordinary IGFET, the Si3 N4 film 3 and SiO2 film 2 are peeled after the selective oxidation, but the Si3 N4 21, 23 formed after the selective oxidation remains. This is mainly because the Si3 N4 21, 23 may not exist as the pure films but they may be complicatedly combined with the impurity particles being contained in the area near the surface of the single crystal silicon substrate 1 as the nuclei. Namely, since the insulated gate film is formed while these nitrides are remaining, a homogeneous film thickness cannot be obtained and resultingly the break down voltage is lowered. In the case of ordinary IGFET, the SiO2 to be provided between the gate electrode and the silicon base plate must be formed as thin as possible in order to make large the electrostatic capacitance at the gate portion. However, when the SiO2 film becomes thinner, the break down voltage is drastically lowered. Lowering of such break down voltage is a large barrier for improvement in the characteristics of the IGFET.
The method of lowering the reaction temperature at the time of selective oxidation is proposed by B. W. Ormont et al. in the Electro-Chemical Society Spring Meeting (Boston) Abstract No. 89 P231 (1979) as the means for solving such lowering of the break down voltage in the insulated gate film due to the selective oxidation.
However, a method of lowering the reaction temperature is inadequate for the actual process because the oxidation time of about 5 hours is usually required for obtaining the field oxide film of about 8000 Å under a temperature of 1100°C but about 13 to 14 hours are required under a temperature of 950°C Moreover, B. W. Ormont et al. also propose a method of using a thick Si3 N4 film as the oxidation resistant mask. But, it is still undesirable measure to make thick the Si3 N4 film enough for preventing lowering of the break down voltage because a stress applied on the base plate at the time of selective oxidation increases.
Therefore, expected is appearing of such an IGFET as not allowing lowering of the break down voltage of the gate insulating film even when the selective oxidation is perpormed using the Si3 N4 film as the mask.
It is an object of the present invention to offer an IGFET which does not allow lowering of the break down voltage of the gate insulating film even when the selective oxidation is carried out using the Si3 N4 film as the mask.
It is another object of the present invention to offer an IGFET which is excellent in the break down voltage even when the gate insulating film is made very thinner and the electrostatic capacitance of the gate portion is made very large.
It is further object of the present invention to offer an IGFET which is excellent in the break down voltage and the function characteristics such as lower threshold voltage and high mutual conductance etc.
It is still further object of the present invention to offer an IGFET which has substantially eliminated the cause of lowered break down voltage of the gate insulating film, does not allow increase of production steps and extension of production period.
The IGFET of the present invention uses the single crystal silicon substrate where the major plain surface is deviated by 22° to 34° from the crystallographic sufface {1,0,0} to {1,1,1} in orientation. In the case of present invention, the gate insulating film is formed on said major plain sufface, the gate electrode is then formed on said gate insulating film as in the case of ordinary IGFET, and the electric field is applied from this gate electrode into said single crystal silicon substrate via said gate insulating film. Moreover, the present invention provides, as in the case of ordinary IGFET, the SiO2 film which is formed on and embedded in said single crystal silicon substrate thicker than the gate insulating film obtained by selectively oxidizing said substrate using the Si3 N4 film as the mask.
More desirably, further improved characteristic can be obtained by forming the epitaxial layer on the single crystal base plate as the abovementioned substrate. As the major plane surface of the gase plate on which the aforementioned opitaxial layer is grown, the crystallographic surface (3,1,1) is particularly excellent in the characteristic among the major plane surfaces of the single crystal silicon base plate indicated within the range of the orientation of the present invention and the adequate thickness of the epitaxial layer is 3 μm or more.
How the objects of the present invention are attained by the IGFET of the present invention will be explained below in detail.
In order to attain such objects, attention is at first focused on the surface orientation of the silicon base plate itself on which the IGFET is formed.
The crystallographic surface orientation is determined by pairs of three figures which are known as the Miller index. Detail explanation about the Miller index is omitted here since it is sufficiently described in the other technical references. However, the Miller constants enclosed by the parentheses indicated a certain specific surface, and the brackets indicate a group of surfaces which are crystallographically equal to said surface.
Usually, the silicon wafer used for an IGFET almost has the major plane surface which is the same as the crystallographic surface {1,0,0}. This is mainly because such condition is required by the number of charges (NFB) induced mainly at the boundary of the single crystal silicon base plate and the gate insulating film and by the mobility (μs) of the carrier in regard to the operation of IGFET at the area near the surface of base plate. Namely, the NFB takes the minimum value at the surface {1,0,0}, while μs takes the maximum value. Therefore, an IGFET having lower threshold voltage and high mutual conductance can be produced.
However, the IGFET in which the surface (1,0,0) is selected as the major plane surface has the lowest break down voltage of gate insulating film as compared with those where the other crystallographi surface is selected as the major plane surface, by executing the selective oxidation using the Si3 N4 film as the mask. Moreover, according to the experimental result of the inventor of the present invention, when the ordinary selective oxidation process is carried out under a temperature of 1100°C, the IGFET formed on a substrate having a surface (1,0,0) as the major plane surface generates so-called "short breakdown" where the gate insulating film breaks down resulting in the perfect conductive condition for the lower voltage applied to the gate electrode. On the other hand, the IGFET formed on the (1,1,1) surface generates so-called "self-heal breakdown" where the gate insulating film once becomes conductive but is immediately cured, even when a voltage which is higher than that for said "short breakdown" by about 10 times is applied.
Explanation for the mechanism of resulting in the low breakdown voltage is already made and therefore omitted here. Explained here is the probable reasons of the low breakdown voltage depending on the crystallographic surface.
Generally, the crystallographic surface is not atomically flat but is so considered as consisting of a kind of the structural step. Since the surface {1,1,1} of the single crystal silicon is considered as the flat surface of step, this step density is also considered to increase as the single crystal silicon surface is deviated toward the surface (1,0,0) from the surface (1,1,1). Since the thermodynamical kink which exists even in the thermally balanced condition is generated mainly in said step, the kink density increases as in the case of said step density when the single crystal silicon surface is deviated toward the surface (1,0,0) from the surface (1,1,1). Particularly, since the kink location is the area where particles are absorbed most easily, most impurity particles are estimated to be absorbed at the surface (1,0,0). As explained above, the low breakdown voltage after selective oxidation is considered to be resulting from nitride generated at the boundary of the silicon base plate due to the reaction between the selective oxidation mask consisting of Si3 N4 and N2 O, and it is proved by variety of experimental results. Since the reaction of producing said nitride proceeds with the impurity particles contained in the area near the single crystal silicon base plate surface working as the nuclei, the amount of nitride generated at the boundary of the single crystal silicon base plate, which ultimately determines the breakdown voltage, depends on the number of impurity particles being absorbed to said single crystal silicon base plate surface. Namely, in case the crystallographic surface {1,1,1} is used as the single crystal silicon base plate surface, the step density at the base plate surface in the atomic range is minimized, the kink density at the surface is also minimized and the amount of impurity particles absorbed to the surface is also minimized, and these values are considered to increase as the surface is inclined toward {1,0,0} crystallographic surface. Moreover, since the chemical reaction indicated by the reaction (2) proceeds depending upon the amount of impurity particles absorbed to the single crystal silicon base plate surface, the least amount of Si3 N4 is formed at the boundary when the crystallographic surface {1,1,1} is used as the single crystal silicon base plate surface and such amount increases as it is deviated gradually toward the crystallographic surface {1,0,0} .
From above explanation, when the selective oxidation is carried out using the Si3 N4 film as the mask, the IGFET has the most excellent breakdown voltage characteristic in case the crystallographic surface {1,1,1} is selected as the single crystal silicon base plate surface and such breakdown voltage is lowered as said surface is deviated toward the surface {1,0,0}.
However, the IGFET which is excellent only in the breakdown voltage of the gate insulating film cannot be said the device having sufficiently excellent characteristics. If an IGRET cannot satisfy the factors which define the element characteristic such as sufficiently small NFB and sufficiently large μs which are also the reason why the single crystal silicon base plate where the abovementioned crystallographic surface (1,0,0) is used as the major plane surface is mainly used, such IGFET cannot be said as the device having excellent overall characteristics. Our experiments for surveying the values of NFB, μs and the breakdown voltage of the gate insulating film after the selective oxidation process was made using the Si3 N4 film as the mask by forming IGFET s on various silicon substrates having crystallographic surface s deviated from (1,0,0) surface to (1,1,1) surface as the major plane surface. These have proved that the factors specifying the characteristics of respective devices do not increase or decrease linearly depending on the deviated angles. Simultaneously, it is also proved that dependence on the inclination angle of base plate of the NFB, μs and breakdown voltage of the gate insulating film respectively show the different characteristics. From above explanation, an IGFET having excellent overall characteristics having respectively the sufficient values of NFB, μs and breakdown voltage within the specific range of angle can be obtained.
First, in regard to the breakdown voltage of the gate insulating film, a sufficiently excellent value is obtained by inclining the surface by 22° or more to the surface (1,1,1) from the surface (1,0,0) with reference to the IGFET using the single crystal silicon base plate formed by the ordinary process where the crystallographic surface (1,0,0) is used as the major plane surface.
Second, sufficient functional characteristics required for a device can be obtained while NFB has a value less than the order of 1010 /cm2, and it has also been proved that such value can be assured when the crystallographic surface is deviated within 34° from the surface (1,0,0) toward the surface (1,1,1).
Third, a particular variation of μs cannot be found in the range of deviation angle of 0° to 45° from the (1,0,0) surface toward the (1,1,1) surface.
On the basis of above explanation, the present invention offers an IGFET using the single crystal silicon base plate having the major plane surface which is inclined only by an angle within the range specified by these factors, namely by the angle from 22° to 34° toward the crystallographic surface {1,1,1} from {1,0,0}. Consequently, the IGFET offered thereby has excellent overall characteristics and particularly assures a high breakdown voltage of the gate insulating film even after the selective oxidation using the Si3 N4 film as the mask.
As explained above, in the present invention, attention is focused at first on the crystallographic surface orientation of the single crystal silicon base plate in view of improving the breakdown voltage of the gate insulating film which is an object of the present invention. Then the inventors of the present invention paied attention to the impurity particles contained in the silicon semiconductor substrate forming an IGFET, particularly existing in the area near the substrate surface. As explained previously, a nitride produced during the selective oxidation using the Si3 N4 film as the mask is the major cause of lowering the breakdown voltage of the gate insulating film, and said impurity particles adhered to the area near the surface of single crystal silicon base plate promotes the chemical reaction producing said nitride. In aforementioned means, the crystallographic surface orientation of the base plate has been determined depending on the amount of impurity particles absorbed to the major plane surface inclined at a certain angle in order to reduce the amount of impurity particles existing at the area near the base plate surface. However, in actual, only a very little amount of impurity particles is absorbed to the crystallographic surface {1,1,1} as compared with other surfaces, but it is not perfectly zero. Therefore, the breakdown voltage of gate insulating film can be more improved by reducing the amount of impurity particles contained in the silicon base plate itself without relation to the crystallographic surface orientation.
Currently, an ordinary IGFET is formed on the single crystal silicon base plate in the bulk condition. However, it is generally known that the single crystal silicon layer obtained by the epitaxial growth contains less amount of impurity particles as compared with the single crystal silicon in the bulk condition. In addition, the epitaxial growth occurs in the same crystal axis as the single crystal silicon base plate. For example, on the base plate where the crystallographic surface (1,0,0) is selected as the major plane surface, the epitaxial layer where the surface (1,0,0) is selected as the major plane surface is formed.
Therefore, less amount of impurity particles are absorbed by deviating the surface toward the surface (1,1,1,) by a certain angle from the surface (1,0,0) of the single crystal silicon substrate, and the same is also true to the epitaxial layer formed on such substrate. Moreover, the epitaxial layer comtains less amount of impurity particles as compared with the substrate in the bulk condition, thereby the single crystal silicon substrate allowing absorption of further less amount of impurity particles can be obtained.
In the case of the present invention, as explained previously, the major plane surface of the single crystal silicon substrate is deviated within the range from 22° to 34° toward the surface (1,1,1) from (1,0,0), and the epitaxial layer is additionally formed on such substrate, thereby further improving the breakdown voltage of the gate insulating film.
It is also known that growth of epitaxial layer which is particularly excellent in the lattice arrangement where the crystal axes are matching with those of the substrate is distinctive at the crystallographic surface of which surface orientation indicated by the Miller index is expressed by integers. Within the abovementioned range, the crystallographic surface {3,1,1} where the major plane surface is deviated by about 25.2° toward the surface {1,1,1} from the surface {1,0,0} is included. Namely, it is desirable to use the substrate having the major plane surface selected to the crystallographic surface {3,1,1} for deviating the crystallographic surface within said specific range and realizing the epitaxial growth.
In case of actually forming an IGFET on the epitaxial layer, the thickness of epitaxial layer becomes a problem.
In the ordinary IGFET, only the area near the substrate surface is related to the operation of device. In addition, in the case of the present invention, a particular thickness is not required since the epitaxial layer is formed in order to improve the breakdown voltage of the gate insulating film after the selective oxidation, namely to reduce the amount of the impurity particles adhered to the substrate surface. However, the crystallization is not so good and unstable at the boundary of single crystal substrate and the epitaxial layer. For this reason, it is desirable for formation of IGFET on the stable epitaxial layer that the epitaxial layer has at least the thickness of 3μ.
FIG. 1 is the cross section of the oxidation resistant mask Si3 N4 film formed on the single crystal silicon substrate via the SiO2 film in the ordinary selective oxidation process.
FIG. 2a is the cross section of substrate under the thermal oxidation using said Si3 N4 film as the mask.
FIG. 2b is the cross section in the process where a nitride is formed at the boundary of the Silicon substrate and the SiO2 film at the mask end of Si3 N4 film of FIG. 2a.
FIG. 3 is the cross section of the substrate where the Si3 N4 film and SiO film of the gate portion are gemoved after the ordinary selective oxidation process.
FIG. 4 shows various results of tests of IGFET using the single crystal silicon substrates having different crystallographic surface orientations. The result A is the breakdown voltage of the gate insulating film, while B is the mobility at the area near the substrate surface and C is the number of charges induced at the boundary of substrate and gate insulating film.
FIG. 5 to FIG. 11 are cross sections of substrate while forming an IGFET using the single crystal silicon substrate of the present invention. Particularly, FIG. 5 to FIG. 8 show the processes: the oxidation resistant Si3 N4 film mask is formed via the SiO2 film, then selective oxidation is performed and thereby a thick SiO2 film is formed.
FIG. 12 is the cross section of base plate where the epitaxial layer is formed on the major plane surface of the single crystal silicon substrate of the present invention.
FIG. 13 is the cross section of an IGFET formed in the same processes indicated in FIG. 5 to FIG. 11 using the base plate indicated in FIG. 12.
An IGFET of the present invention will be explained by referring to FIG. 4. In FIG. 4, A is the result of breakdown voltage test of the gate insulating film. Here used as the single crystal silicon substrate are the N or P conductive type substrates and the selective oxidation has been carried out using the Si3 N4 film as the mask before formation of the gate insulating film. In the selective oxidation, the field oxide film in the thickness of about 7800 Å is formed under a temperature of about 1100°C which is employed for ordinary oxidation process. Here, the thickness of SiO2 film formed on the entire part of the major plane surface of the silicon substrate before the selective oxidation process is about 500 Å, while the thickness of the Si3 N4 film is about 500 Å. After the selective oxidation process, the Si3 N4 film and the SiO2 film being formed on the gate portion are removed from the substrate surface and then the SiO2 film is newly formed in the thickness of about 1000 Å as the gate insulating film.
For the breakdown voltage test, the electrical field up to 5 (MV/cm) is applied, which causes perfect breakdown and brings about the conductive condition to said gate insulating film of the substrate where the crystallographic surface (1,0,0) is selected as the major plane surface. The vertical axis of FIG. 4 shows the percentage of all samples in the same kind subjected to the test. The horizontal axis indicates an deviation angle of the crystallographic surface toward the surface (1,1,1) from (1,0,0) in unit of degree.
The breakdown voltage failure rate of this gate insulating film is minimum and all samples have generated the breakdown. On the other hand, such failure rate is maximum at the crystallographic surface (1,1,1) inclined by about 54.7° from the surface (1,0,0), and all samples do not generate the breakdown. Difference in the breakdown voltage depending on the conductive type of substrate could not be found. Dependence on inclination angle of the crystallographic surface of the breakdown voltage failure rate is gradually improved within the range of inclination angle from 10° to 40°.
It can be generally said that the lowering of breakdown voltage of the gate insulating film is obviously prevented by obtaining the breakdown voltage failure rate of 50% or less with the electrical field as high as 5 MV/cm which causes the breakdown of 100% at the crystallographic surface (1,0,0) being selected in the ordinary IGFET. From the FIG. 4, the breakdown voltage failure rate of 50% is obtained at the point where the crystallographic surface is deviated by 22° toward the surface (1,1,1,) from the surface (1,1,1,) from the surface (1,0,0). In this point, the gate insulating film consisting of SiO2 in the thickness of about 1000 Å little generates breakdown for application of the gate voltage of about 30 V or less, and resultingly an IGFET which is excellent in the breakdown voltage characteristic can be obtained.
The result C in FIG. 4 indicates dependence on the crystallographic surface orientation of the number of charges NFB induced at the boundary of the single crystal silicon substrate and the SiO2 film used as the gate insulating film. This is the result of test using the SiO2 film formed by the same process as that for said breakdown voltage test as the gate insulating film. Here, the vertical axis indicates the values of crystallographic surfaces in the unit of percentage where the value of the surface (1,1,1,) is indicated as 100%. The values of NFB increases when the angle of crystallographic surface orientation is deviated by 20° or more from the surface (1,0,0). The NFB value determines the boundary level and is the factors for determining many functional characteristics such as threshold voltage and mutual conductance etc. Therefore, the NFB value becomes 1011 /cm2 or more when the surface is deviated by 34° or more toward the surface (1,1,1) from (1,0,0) and an element being superior in the functional characteristics cannot be obtained.
The result B in FIG. 4 indicates dependence on the crystallographic surface orientation of the mobility μs of carrier in the area being related to the operation of IGFET near the silicon substrate surface. The vertical axis indicates the values at respective crystallographic surfaces in the unit of percentage where the value at the crystallographic surface (1,0,0) is indicated as 100%. The mobility μs little changes until the inclination angle of the crystallographic surface reaches about 45°.
From the results A to C in FIG. 4, namely from respective requirements of breakdown voltage of the gate insulating film, mobility μs and number of charges NFB at the boundary, the present invention proposes that the major plane surface of the single crystal silicon substrate used for IGFET is deviated within the range from 22° to 34° toward the crystallographic surface {1,1,1,} from {1,0,0}.
Moreover, it is desirable in the present invention to allow the epitaxial growth on the single crystal silicon substrate. In regard to the factors such as NFB, μs which specify the functional characteristics of an IGFET indicated in FIG. 4, distinctive change cannot be found even when the epitaxial growth is performed. But the breakdown voltage failure rate of the gate insulating film is very much improved as a whole by providing the epitaxial layer. Therefore, in order to further improve the breakdown voltage characteristic of an IGFET, the epitaxial layer is grown on the single crystal silicon substrate where the major plane surface is deviated by 22° to 34° toward the crystallographic surface {1,1,1,} from {1,0,0}. Moreover, the epitaxial layer having good crystallization can be obtained by using such a substrate where the crustallographic surface within said ranged, particularly the surface {3,1,1,} is used as the major plane surface as the substrate for epitaxial layer. In addition, the adequate thickness of the epitaxial layer is 3 μm or more from the abovementioned reason, and a thicker epitaxial layer obtained by the epitaxial growth will not provide any particular improvement in the element characteristic and only makes longer the period of time required by the processes. Thus, in the actual process, the epitaxial layer in the thickness of about 3μ is obtained by epitaxial growth.
The preferred IGFET forming process of the present invention will be explained below by referring to the drawings.
At first, FIG. 5 to FIG. 11 respectively show the formation of IGFET using the substrate where the major plane surface is deriated.
FIG. 5 shows the profile where the SiO2 film 32 and Si3 N4 film 33 are respectively formed on the single crystal silicon substrate.
Prepared, first of all, is the single crystal silicon substrate 31 having the p type conductivity, specific resistance of about 10 ohms, and thickness of about 600 μm where the crystallographic surface which is deviated by 25° toward the surface {1,1,1,} from {1,0,0} is selected as the major plane surface. Thereafter, the major plane surface of said single crystal silicon substrate 31 is totally oxidized under the oxygen ambient containing HCl of about 5%. After the oxidation process for about 30 min., the SiO2 film 32 of about 500 Å can be obtained. Then, the Si3 N4 film 33 is formed on said SiO2 film by the ordinary CVD (Chemical Vapor Deposition) method. The growth of Si3 N4 is carried out at a temperature of about 800°C under the ambient of SiH4 :NH3 =1:50 as the typical epipaxial growth condition and thereby the thickness of about 500 Å can be obtained.
FIG. 6 shows the profile where only the Si3 N4 film 33 among said SiO2 film 32 and Si3 N4 film 33 is remained only on the IGFET forming area. As the first step, the resist is coated on the entire part of the major plane surface of said single crystal silicon substrate 31 and the IGFET forming region is developed by patterning. Succeedingly, the Si3 N4 film 33 only on the said IGFET forming area is remained but that on the other area is removed by means of a conventional plasma etching method. Moreover, as in the case of ordinary IGFET, the boron ion in amount of about 3.5×1013 /cm2 is injected with an energy of about 40 KeV into the major plane surface of the substrate using said remaining Si3 N4 film 33 as the mask in order to form the channel cut region.
FIG. 7 shows the profile where a thick SiO2 film 35 is formed using the Si3 N4 film 33 as the mask by the thermal oxidation process. Since the Si3 N4 film shows excellent oxidation resistant characteristic, it is used as the oxidation mask in the ordinary selective oxidation proces. This Si3 N4 film has a problem mentioned previously, but here the Si3 N4 film is imevitably used as the oxidation mask since the alternative cannot still be obtained.
Typically, on the region from where the Si3 N4 film has been removed, the SiO2 film 35 as thick as about 7800 Å is formed by the thermal oxidation form about 4 hours at a temperature of about 1100°C under the wet oxygen (O2) ambient (containing water vapor). Simultaneously, the boron ion injected precedingly becomes active due to said anneal process, thus forming the p+ region 34 which will become the channel cut region.
FIG. 8 shows the profile where the thin SiO2 film 32 is removed from said Si3 N4 film 33 and the IGFET forming region.
At first, the SiO2 being formed on the area near the surface of said Si3 N4 film 33 is perfectly removed using the buffer-HF as the etching solution. Next, said Si3 N4 film 33 is removed from the surface of substrate using hot-H3 PO4 as the etching solution. Moreover, a comparatively thin SiO2 film 32 on the IGFET forming region is removed using again the buffer-HF. Here, in the ordinary single crystal silicon substrate where the crystallographic surface (1,0,0) is selected as the major plane surface, a nitride newly produced by the selective oxidation process is still remaining even after said etching in said IGFET forming region, but in the case of using the substrate of the present invention where the major plane surface is selected to the crystallographic surface deviated by 25° toward the surface {1,1,1,} from the surface {1,0,0}, nitride compound is not substantially formed at the boundary of said single crystal silicon substrate, and is not remained as indicated in FIG. 8 even after removery of SiO2 film.
FIG. 9 shows the profile where the gate insulating film is formed on the single crystal silicon substrate 31.
The gate SiO2 film 36 is formed in the thickness of about 500 Å on said IGFET forming region under the oxygen ambinet of about 1100° C. containing HCl of about 5%. Here, since the boundary of the single crystal silicon substrate 31 and the SiO2 film 36 is very stable as indicated in FIG. 8, a good SiO2 film 36 having homogeneous thickness can be formed.
FIG. 10 shows the profile where the n+ source and drain regions 37 are provided separately within said single crystal silicon substrate 31 and the gate electrode 38 consisting of poly-silicon is formed on said gate SiO2 film 36.
At first, the poly-silicon is formed by the CVD method on the entire part of the major plane surface of substrate, then the resist film is coated on the surface of said poly-silicon film, patterning is performed only to the gate electrode forming region, and the gate electrode 38 is formed by etching only the poly-silicon film with the nitric or fluoric acid. On the other hand, the ion implantation is carrierd out only to the area where a thin SiO2 film 36 is formed on the single crystal silicon substrate using said gate electrode 38 as the mask. Here, as the ions, the As ion in amount of 4×1015 /cm2 is implanted with an energy of about 120 KeV. Thereafter, the As ion implanted previously is activated, thereby forming the source/drain region 37 having the n+ type conductivity.
FIG. 11 shows the profile where an IGFET is completed with disposition of the electrodes from said source/drain region 37.
First, the PSG film 39 is formed on the entire part of the major plane surface of substrate by the CVD method using SiH4, PH4 and O2 gasses. Succeedingly, the boring is performed by the etching method to said PSG film 39 and thin SiO2 film 36 for the contact with the source/drain regions 37. Moreover, the aluminium electrode 40 is evaporated, thus completing an IGFET.
From FIG. 5 to FIG. 11, the process of forming an IGFET using the single crystal silicon substrate of the present invention is indicated. FIG. 12 and FIG. 13 shows the process of forming an IGFET using the epitaxial layer of the present invention.
FIG. 12 shows the silicon substrate of the present invention where the epitaxial layer is formed. The p type epitaxial layer 42 which is the same as the substrate in the conductivity type and has the specific rasistance of 10 ohms is formed in the thickness of about 3 μm on the single crystal silicon substrate where the crystallographic surface (3,1,1) is selected as the major plane surface. Here, the crystallographic surface orientation of the major plane surface of the opitaxial layer is also (3,1,1) as in the case of the substrate 41.
FIG. 13 shows the profile where an IGFET is completed in said epitaxial layer.
With the same process explained in regard to said FIG. 5 to FIG. 11, an IGFET is formed in said epitaxial layer 42. Here, 44 is the P+ type channel cut region; 45 is the thick SiO2 film; 46 is the thin SiO2 film of the gate portion; 47 is the n+ type source/drain region; 48 is the gate electrode of poly-silicon; 49 is the PSG film, and 50 is the aluminium electrode, respectively.
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